As the opening of the once-in-a-decade international stamp exhibition, Ameripex 86, approaches, the philatelic celebration of the largest stamp show ever held in North America keeps mounting.

The United States is adding a second first-day-of-issue ceremony for a 14-cent postal card that is a tribute to stamp collectors and stamp collecting. The ceremony will take place on the second day of the show, which runs from May 22 to June 1 at the O'Hare Exposition Center in Rosemont, Ill.

The opening day of the show will have the first-day ceremony for the major U.S. issue connected with the show, four sheetlets of nine 22-cent stamps picturing the 35 deceased presidents, with the White House making up the 36th stamp.

In addition, the United States is joining up with Sweden and the United Nations in putting out a folder entitled "Stamp Collecting -- The International Hobby." Later this year the Swedes are staging Stockholmia 86, the once-in-a-decade European exhibition.

The United States and Sweden got together at the beginning of this year in a joint issue for four stamps each, with the third stamp of each quartet in a common design. Sweden's issue heralded its upcoming show. The American commemoratives celebrated stamp collecting, but there were ample references to Ameripex.

There is still more. On the opening day of Ameripex the Postal Service is putting out a 28-page full-color booklet containing a full set of the presidential sheetlets along with illustrations and commentary about each chief executive.

Instead of focusing on Americana, the added Ameripex postal card takes a wider view of philately, taking as its text "Stamps, the Universal Hobby." The design of the card's imprinted stamp depicts the silhouetted figures of three men and three women with one youngster sandwiched among them. All are examining and discussing stamps while standing before a dealer's table covered with stamps, albums and collecting paraphernalia.

The design is based on a paper sculpture created by Ray Ameijide of Peekskill, N.Y.

Collectors of first-day cancellations have the customary 30-day grace period for ordering -- orders must be postmarked no later than June 23 -- and alternative ways to order.

Collectors acquiring cards themselves should send them for cancellation to Customer-Provided Postal Cards, Postmaster, Chicago, Ill. 60607-9992. The cost is 14 cents per card. Personal checks are accepted, cash is not welcomed, payment by postage stamps is refused. The use of return labels is requested.

The joint philatelic folder ly 5 3/4 by 8 1/4 inches. The first page contains the four Stamp Collecting booklet stamps put out by the United States in January, canceled with the Presidents Day pictorial cancellation that will be used at Ameripex on May 22. Background information is in English. The information appears in Swedish, French and German on the reverse side of the page.

Another page is devoted to the four Swedish stamps issued jointly with the United States in January. Background information is in Swedish; English, French and German translations are on the reverse side of the page.

The third page depicts six stamps being issued by the United Nations in honor of stamp collecting, with a text in English, French, German and Swedish on the reverse.

The folder is being sold for $5.95 at Ameripex, and thereafter at selected philatelic centers and by mail order from the Philatelic Sales Division, Washington, D.C. 20265-9998 from May 22 through Sept. 7, the closing date for Stockholmia. The folder will also be available from the U.N. Postal Administration and the Swedish Philatelic Service.

The booklet is priced at $10 and will be sold at Ameripex and at post offices throughout the country.

The deadline for ordering first-day cancellations of the $2 William Jennings Bryan regular issue has been extended an additional 30 days. The new deadline is May 18.